1. Field of the Invention
The invention relates to a solar-thermal conversion member, a solar-thermal conversion device, and a solar thermal power generation device.
2. Description of Related Art
Conventional solar thermal power generation devices convert sunlight to heat and use this heat to generate power. In these devices, sunlight is concentrated at a collector and this concentrated sunlight is used to heat a thermal medium (oil, dissolved salt, molten sodium, and so forth) in a container or a flow channel.
Investigations have been carried out into coating the surface of the container or flow channel in order with this coating to promote the absorption of the concentrated sunlight and inhibit thermal radiation by thermal emission to the exterior from the container or flow channel (July, 2002, NREL/TP-520-31267, “Review of Mid- to High-Temperature Solar Selective Absorber Materials”, C. E. Kennedy (referred to below as Kennedy)).
In connection with this, as shown in FIG. 1 the solar spectrum (thermal emission spectrum at a blackbody radiation temperature of approximately 5500° C.) extends centered on the visible light region at wavelengths of several hundred nm. On the other hand, the thermal emission spectrum at several hundred ° C. (for example, approximately 580° C.), which are temperatures readily obtained in solar-thermal conversion devices, extends centered on the infrared region at wavelengths of several thousand nm. Thus, the range of the thermal emission spectrum at the temperatures obtained in solar-thermal conversion devices is shifted from the range of the solar spectrum.
The emittance of thermal emission at a particular temperature corresponds to the absorptance for the light in the thermal emission spectrum corresponding to this temperature. Accordingly, low thermal radiation due to thermal emission at a temperature of several hundred ° C. means a small absorptance for the light in the thermal emission spectrum corresponding to a temperature of several hundred ° C., i.e., a small absorptance for infrared light at wavelengths of several thousand nm.
Thus, a coating that exhibits a high absorptance for sunlight and a low thermal radiation due to thermal emission at a temperature of several hundred ° C. can be said to be a coating that has a large absorptance for visible light at wavelengths of several hundred nm and a small absorptance for infrared light at wavelengths of several thousand nm. Such a coating can be advantageously used to promote the absorption of concentrated sunlight and to inhibit thermal radiation due to thermal emission to the outside from a container or flow channel.
Kennedy lists materials for such coatings and specifically gives W, MoO3-doped Mo, B-doped Si, CaF2, HfC, ZrB2, SnO2, In2O3, Eu2O3, ReO3, V2O5, LaB6, and so forth.
In addition to the selection of the material itself for such coatings, the layer structure of the coating is also conventionally optimized.
Coating layer structures are specifically described, for example, by Kennedy and in WO 02/103257. These documents also propose that interference due to reflection at the interfaces of a plurality of stacked layers having different refractive indices can be used to provide a coating that exhibits a high absorptance for visible light at wavelengths of several hundred nm and a low absorptance for infrared light at wavelengths of several thousand nm. Here, Kennedy describes the use, for example, of stacks of metal layers such as Mo, Ag, Cu, and Ni with dielectric layers such as Al2O3, SiO2, CeO2, and ZnS.